1. Field of the Invention
The present invention relates to rotation control of a polyphase motor.
2. Description of the Background Art
A servo motor is used for high-precision rotation control and position control as a spindle motor used for audio devices and computer peripheral equipment (for example, see FIG. 3 of Japanese Patent Laying-Open No. 2007-074816). FIG. 5 is a block diagram schematically showing a configuration of a conventional spindle apparatus. In the following, referring to FIG. 5, the configuration of the conventional spindle apparatus is described.
In FIG. 5, the spindle apparatus is configured by a spindle motor 131 and a drive control circuit 132 therefor. Spindle motor 131 includes a motor body 133 being a three-phase brushless motor, an encoder 134 fixed to the spindle shaft of motor body 133, and the like. Encoder 134 detects the rotation speed, rotor position and the like of motor body 133, and feeds back a detection signal to drive control circuit 132.
Drive control circuit 132 in FIG. 5 includes a control computer 141 and drive circuit 142. In response to a current command value provided by control computer 141, drive circuit 142 outputs armature currents of three-phase sine waves of U, V, W phases passing through armature windings of motor body 133. When motor body 133 is driven by pulse width modulation (PWM), drive circuit 142 is configured by three-phase inverter circuitry. The U- and W-phase armature currents are detected by current detectors 137a, 137b and digitally converted by A/D converters 138a, 138b, respectively, and thereafter provided as feedback signals to control computer 141. Here, the V-phase armature current can be determined based on that the sum of U, V, W phases is 0. It is to be noted that, while control computer 141 is used in drive control circuit 132 in FIG. 5, the equivalent function can be configured using analog circuitry.
For the control of a spindle motor for which particularly high-precision rotation speed control is required, such as a spindle motor used in manufacture/inspection processes of magnetic disks and optical disks or in precision processing machines and precision inspection devices, PLL (Phase Lock Loop) control is employed (for example, see Japanese Patent Laying-Open No. 2002-078374). In the spindle device of PLL control, a reference pulse signal generated by a crystal oscillator or the like and a pulse signal corresponding to the rotation speed of a motor body detected by an encoder are compared with each other to obtain the phase difference, whereby a current command value is determined.
In the above-described use, capability of driving at a constant speed for a certain period is important as a basic performance of the spindle motor. In particular, as to the driving at a constant speed, it is required that rotation fluctuation, vibration and noise are small.
One cause of the rotation fluctuation and vibration may be that, in a three-phase motor, the amplitudes of sine waves passing through the U-, V-, and W-phase armature windings are varied and not uniform. When there is variation in the amplitude among the armature currents of respective phases, ripple occurs in the torque of the motor, and therefore the motor vibrates. The number of such vibration is proportional to the number of poles of the rotor. The vibration caused by variation in the amplitude among the armature currents of respective phases may occur not only in the three-phase motor, but in any polyphase motor.
Such variation in the amplitude among the armature currents of respective phases is considered to be caused by variation in the winding resistance among the armature windings of the motor body, variation in the characteristics among power devices used in the drive circuit and the like. When the control circuit for the motor is configured by analog circuitry, the cause may be variation in the characteristics of operational amplifiers, resistors and the like. In particular, in a case of a servo motor that performs feedback control, variations in the characteristics of elements used in the drive control circuit may leave offset and phase shift between a current command value and an actual armature current after a steady state is entered.
Current scheme for reducing variation among the armature currents of respective phases is to select resistors, operation amplifiers, power devices and the like used in the drive control circuit for the motor so that variation among them is small. As to the motor body also, the one in which variation in winding resistance is small is selected. Accordingly, a plurality of these components must be prepared, whereby extra costs and trouble are required.